In Vivo and In Vitro Characterization of the Novel Antiarrhythmic Agent SSR149744C

Novel Arylpiperazine Derivatives of Salicylamide with α1-Adrenolytic Properties Showed Antiarrhythmic and Hypotensive Properties in Rats

Cardiovascular diseases remain one of the leading causes of death worldwide. Unfortunately, the available pharmacotherapeutic options have limited effectiveness. Therefore, developing new drug candidates remains very important. We selected six novel arylpiperazine alkyl derivatives of salicylamide to investigate their cardiovascular effects. Having in mind the beneficial role of α₁-adrenergic receptors in restoring sinus rhythm and regulating blood pressure, first, using radioligand binding assays, we evaluated the affinity of the tested compounds for α-adrenergic receptors. Our experiments revealed their high to moderate affinity for α₁- but not α₂-adrenoceptors. Next, we aimed to determine the antiarrhythmic potential of novel derivatives in rat models of arrhythmia induced by adrenaline, calcium chloride, or aconitine. All compounds showed potent prophylactic antiarrhythmic activity in the adrenaline-induced arrhythmia model and no effects in calcium chloride- or aconitine-induced arrhythmias. Moreover, the tested compounds demonstrated therapeutic antiarrhythmic activity, restoring a normal sinus rhythm immediately after the administration of the arrhythmogen adrenaline. Notably, none of the tested derivatives affected the normal electrocardiogram (ECG) parameters in rodents, which excludes their proarrhythmic potential. Finally, all tested compounds decreased blood pressure in normotensive rats and reversed the pressor response to methoxamine, suggesting that their hypotensive mechanism of action is connected with the blockade of α₁-adrenoceptors. Our results confirm the antiarrhythmic and hypotensive activities of novel arylpiperazine derivatives and encourage their further investigation as model structures for potential drugs.

Challenges Faced with Small Molecular Modulators of Potassium Current Channel Isoform Kv1.5

The voltage-gated potassium channel Kv1.5, which mediates the cardiac ultra-rapid delayed-rectifier (IKur) current in human cells, has a crucial role in atrial fibrillation. Therefore, the design of selective Kv1.5 modulators is essential for the treatment of pathophysiological conditions involving Kv1.5 activity. This review summarizes the progress of molecular structures and the functionality of different types of Kv1.5 modulators, with a focus on clinical cardiovascular drugs and a number of active natural products, through a summarization of 96 compounds currently widely used. Furthermore, we also discuss the contributions of Kv1.5 and the regulation of the structure-activity relationship (SAR) of synthetic Kv1.5 inhibitors in human pathophysiology. SAR analysis is regarded as a useful strategy in structural elucidation, as it relates to the characteristics that improve compounds targeting Kv1.5. Herein, we present previous studies regarding the structural, pharmacological, and SAR information of the Kv1.5 modulator, through which we can assist in identifying and designing potent and specific Kv1.5 inhibitors in the treatment of diseases involving Kv1.5 activity.

German Cardiac Society Working Group on Cellular Electrophysiology state-of-the-art paper: impact of molecular mechanisms on clinical arrhythmia management

Cardiac arrhythmias remain a common challenge and are associated with significant morbidity and mortality. Effective and safe rhythm control strategies are a primary, yet unmet need in everyday clinical practice. Despite significant pharmacological and technological advances, including catheter ablation and device-based therapies, the development of more effective alternatives is of significant interest to increase quality of life and to reduce symptom burden, hospitalizations and mortality. The mechanistic understanding of pathophysiological pathways underlying cardiac arrhythmias has advanced profoundly, opening up novel avenues for mechanism-based therapeutic approaches. Current management of arrhythmias, however, is primarily guided by clinical and demographic characteristics of patient groups as opposed to individual, patient-specific mechanisms and pheno-/genotyping. With this state-of-the-art paper, the Working Group on Cellular Electrophysiology of the German Cardiac Society aims to close the gap between advanced molecular understanding and clinical decision-making in cardiac electrophysiology. The significance of cellular electrophysiological findings for clinical arrhythmia management constitutes the main focus of this document. Clinically relevant knowledge of pathophysiological pathways of arrhythmias and cellular mechanisms of antiarrhythmic interventions are summarized. Furthermore, the specific molecular background for the initiation and perpetuation of atrial and ventricular arrhythmias and mechanism-based strategies for therapeutic interventions are highlighted. Current "hot topics" in atrial fibrillation are critically appraised. Finally, the establishment and support of cellular and translational electrophysiology programs in clinical rhythmology departments is called for to improve basic-science-guided patient management.

Mechanisms and Drug Development in Atrial Fibrillation

Atrial fibrillation is a highly prevalent cardiac arrhythmia and the most important cause of embolic stroke. Although genetic studies have identified an increasing assembly of AF-related genes, the impact of these genetic discoveries is yet to be realized. In addition, despite more than a century of research and speculation, the molecular and cellular mechanisms underlying AF have not been established, and therapy for AF, particularly persistent AF, remains suboptimal. Current antiarrhythmic drugs are associated with a significant rate of adverse events, particularly proarrhythmia, which may explain why many highly symptomatic AF patients are not receiving any rhythm control therapy. This review focuses on recent advances in AF research, including its epidemiology, genetics, and pathophysiological mechanisms. We then discuss the status of antiarrhythmic drug therapy for AF today, reviewing molecular mechanisms, and the possible clinical use of some of the new atrial-selective antifibrillatory agents, as well as drugs that target atrial remodeling, inflammation and fibrosis, which are being tested as upstream therapies to prevent AF perpetuation. Altogether, the objective is to highlight the magnitude and endemic dimension of AF, which requires a significant effort to develop new and effective antiarrhythmic drugs, but also improve AF prevention and treatment of risk factors that are associated with AF complications.

Antiarrhythmic drugs 2013: state of the art

Antiarrhythmic drugs are widely used, but are of modest efficacy and have important side effects. However, even with the advance of catheter ablation for atrial fibrillation and ventricular tachycardia, antiarrhythmic drugs remain an important tool for treating arrhythmias. Antiarrhythmic drug development has remained slow despite much effort given our limited understanding of what role various ionic currents play in arrhythmogenesis and how they are modified by arrhythmias. This review will focus on promising new antiarrhythmic drugs undergoing clinical investigation or currently approved for clinical use, including amiodarone analogues, agents with novel ionic targets, and new drug combinations.

Safety and efficacy of vernakalant for acute cardioversion of atrial fibrillation: an update

Intravenous vernakalant has recently been approved in Europe as an atrial-selective antiarrhythmic drug for the conversion of recent-onset atrial fibrillation (AF). It inhibits atrial-selective K⁺ currents (I_K,ACh and I_Kur) and causes rate-dependent atrial-predominant Na⁺ channel block, with only a small inhibitory effect on the rapid delayed rectifier K⁺ current (I_Kr) in the ventricle. Due to its atrial-selective properties, vernakalant prolongs the effective refractory period of the atria with minimal effects on the ventricles, being associated with a low proarrhythmic risk for torsades de pointes arrhythmias. Five pivotal clinical trials consistently demonstrated that vernakalant rapidly terminates AF with stable maintenance of sinus rhythm for up to 24 hours. A head-to-head comparative trial showed that the 90-minute conversion rate of vernakalant was substantially higher than that of amiodarone. Initially, a longer-acting oral formulation of vernakalant was shown to be effective and safe in preventing AF recurrence after cardioversion in a Phase IIb study. However, the clinical studies testing oral vernakalant for maintenance of sinus rhythm after AF cardioversion were prematurely halted for undisclosed reasons. This review article provides an update on the safety and efficacy of intravenous vernakalant for the rapid cardioversion of AF.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

Slow delayed rectifying potassium current (IKs ) - analysis of the in vitro inhibition data and predictive model development

The excitable cell membranes contain ion channels that allow the ions passage through the specific pores via a passive process. Assessment of the inhibition of the IKr (hERG) current is considered to be the main target during the drug development process, although there are other ionic currents for which drug-triggered modification can either potentiate or mask hERG channel blockade. Information describing the results of in vitro studies investigating the chemical-IKs current interactions has been developed in the current study. Based on the publicly available data sources, 145 records were collected. The final list of publications consists of 64 positions and refers to 106 different molecules connected with IKs current inhibition, with at least one IC50 value measured. Ultimately, 98 of the IC50 values expressed as absolute values were gathered. For 36 records the IC50 was expressed as a relative value. For the 11 remaining records, the inhibition was not clearly expressed. Based on the collected data the predictive models for the IC50 estimation were developed with the use of various algorithms. The extended Quantitative Structure-Activity Relationships (QSAR) methodology was applied and the in vitro research settings were included as independent variables, apart from the physico-chemical descriptors calculated with the use of the Marvin Calculator Plugins. The root mean squared error and normalized root mean squared error values for the best model (an expert system based on two independent artificial neural networks) were 0.86 and 14.04%, respectively. The model was further built into the ToxComp system, the ToxIVIVE tool specialized for cardiotoxicity assessment of drugs.

Efficacy and safety of celivarone, with amiodarone as calibrator, in patients with an implantable cardioverter-defibrillator for prevention of implantable cardioverter-defibrillator interventions or death: the ALPHEE study

BACKGROUND

Celivarone is a new antiarrhythmic agent developed for the treatment of ventricular arrhythmias. This study investigated the efficacy and safety of celivarone in preventing implantable cardioverter-defibrillator (ICD) interventions or death.

METHODS AND RESULTS

Celivarone (50, 100, or 300 mg/d) was assessed compared with placebo in this randomized, double-blind, placebo-controlled, parallel-group study. Amiodarone (200 mg/d after loading dose of 600 mg/d for 10 days) was used as a calibrator. A total of 486 patients with a left ventricular ejection fraction ≤40% and at least 1 ICD intervention for ventricular tachycardia or ventricular fibrillation in the previous month or ICD implantation in the previous month for documented ventricular tachycardia/ventricular fibrillation were randomized. Median treatment duration was 9 months. The primary efficacy end point was occurrence of ventricular tachycardia/ventricular fibrillation-triggered ICD interventions (shocks or antitachycardia pacing) or sudden death. The proportion of patients experiencing an appropriate ICD intervention or sudden death was 61.5% in the placebo group; 67.0%, 58.8%, and 54.9% in the celivarone 50-, 100-, and 300-mg groups, respectively; and 45.3% in the amiodarone group. Hazard ratios versus placebo for the primary end point ranged from 0.860 for celivarone 300 mg to 1.199 for celivarone 50 mg. None of the comparisons versus placebo were statistically significant. Celivarone had an acceptable safety profile.

CONCLUSIONS

Celivarone was not effective for the prevention of ICD interventions or sudden death.

CLINICAL TRIAL REGISTRATION

http://www.clinicaltrials.gov. Unique identifier: NCT00993382.

Celivarone in patients with an implantable cardioverter-defibrillator: adjunctive therapy for the reduction of ventricular arrhythmia-triggered implantable cardioverter-defibrillator interventions

BACKGROUND

Implantable cardioverter-defibrillators (ICDs) remain the treatment of choice for the prevention of life-threatening arrhythmias. However, many patients with ICDs require additional antiarrhythmic therapy to reduce the morbidity associated with recurrent arrhythmia-triggered ICD interventions.

OBJECTIVE

Our study aimed to evaluate the safety and efficacy of celivarone in reducing these interventions.

METHODS

A total of 153 eligible ICD recipients were randomized to receive either placebo or celivarone 100 or 300 mg once daily for 6 months. The primary end point was the prevention of arrhythmia-triggered ICD therapies.

RESULTS

Fewer ventricular tachycardia and ventricular fibrillation episodes were observed in the 300-mg celivarone group than in the placebo group, with a relative risk reduction of 46%, which was not statistically significant. The analysis of all-cause shocks showed a trend toward a decreased number of events in the celivarone 300-mg group. A post hoc analysis of the primary end point in a subgroup of patients in the celivarone 300-mg group, who had received ICD therapy within 1 month of randomization, showed a significant benefit (P = .032). Celivarone was not associated with an increased risk of torsades de pointes, thyroid dysfunction, or pulmonary events. More heart failure events were reported in the celivarone groups than in the placebo group, but the difference was not statistically significant.

CONCLUSION

Celivarone tends to reduce ventricular tachycardia-/ventricular fibrillation-triggered ICD therapies. This effect was not statistically significant. There was a trend toward greater efficacy in the 300-mg group, especially in patients undergoing ICD therapy within 30 days prior to randomization. Overall, celivarone was well tolerated.

Combined blockade of early and late activated atrial potassium currents suppresses atrial fibrillation in a pig model of obstructive apnea

BACKGROUND

Negative tracheal pressure (NTP) during tracheal obstruction in obstructive apnea increases vagal tone and causes pronounced shortening of the atrial effective refractory period (AERP), thereby perpetuating atrial fibrillation (AF). The role of different atrial potassium channels under those conditions has not been investigated.

OBJECTIVE

The purpose of this study was to evaluate the atrial effects of blockade of the late activated potassium current (I(Kr)) by sotalol, of blockade of the early activated potassium currents (I(Kur)/I(to)) by AVE0118, and of the multichannel blocker amiodarone during tracheal occlusions with applied NTP.

METHODS

Twenty-one pigs were anesthetized, and an endotracheal tube was placed to apply NTP (up to -100 mbar) comparable to clinically observed obstructive sleep apnea for 2 minutes. Right AERP and AF inducibility were measured transvenously by a monophasic action potential recording and stimulation catheter.

RESULTS

Tracheal occlusion with applied NTP caused pronounced AERP shortening. AF was inducible during all NTP maneuvers. Blockade of I(Kr) by sotalol, blockade of I(Kur)/I(to) by AVE0118, and amiodarone did not affect NTP-induced AERP shortening, although they prolonged the AERP during normal breathing. Atropine given after amiodarone completely inhibited NTP-induced AERP shortening. The combined blockade of I(Kr) and I(Kur)/I(to) by sotalol plus AVE0118, however, attenuated NTP-induced AERP shortening and AF inducibility independent of the order of administration.

CONCLUSION

The atrial proarrhythmic effect of NTP simulating obstructive apneas is difficult to inhibit by class III antiarrhythmic drugs. Neither amiodarone nor blockade of I(Kr) or I(Kur)/I(to) attenuated NTP-induced AERP shortening. However, the combined blockade of I(Kur)/I(to) and I(Kr) suppressed NTP-induced AERP shortening.

Effects of the novel amiodarone-like compound SAR114646A on cardiac ion channels and ventricular arrhythmias in rats

Amiodarone is the "gold standard" for current antiarrhythmic therapy because it combines efficacy with good hemodynamic and electrophysiological tolerance. Amiodarone is effective against both atrial and ventricular arrhythmias by intravenous (i.v.) or oral route. However, the i.v. formulation has limitations. Therefore, we identified SAR114646A, an amiodarone-like antiarrhythmic agent with good aqueous solubility suitable for i.v. application. Patch-clamp experiments were performed with isolated cardiomyocytes from guinea pigs and rats. In guinea pig ventricular cardiomyocytes, the fast Na(+) channel and the L-type Ca(2+) channels were blocked by SAR114646A with half-maximal concentrations (IC(50)) of 2.0 and 1.1 μM, respectively. The tail current of the fast activating rectifying potassium channel I(Kr) was blocked with an IC(50) value of 0.6 μM, whereas the IC(50) values for inhibition of the I(Ks) and I(K1) channels was >10 μM. ATP-sensitive K(+) channels were evoked by application of the channel opener rilmakalim (3 μM). SAR114646A blocked this current with an IC(50) value of 2.8 μM. In guinea pig atrial cardiomyocytes, carbachol (1 μM) was used to activate the I(KACh) and SAR114646A inhibited this current with IC(50) of 36.5 nM. The transient outward current I(to) and the sustained current I(sus) were investigated in rat ventricular myocytes. SAR114646A blocked these currents with IC(50) of 1.8 and 1.2 μM, respectively. When expressed in Chinese hamster ovary cells, the currents hKv1.5 and hHCN4 were inhibited with IC(50) values of 1.1 and 0.4 μM, respectively. Micropuncture experiments in isolated rabbit left atria revealed that SAR114646A prolonged the 50% repolarization significantly at 3 and 10 μM. In guinea pig papillary muscle, the APD at 90% of repolarization was slightly prolonged at 3 and 10 μM. SAR114646A demonstrates antiarrhythmic activity in anaesthetised rats, subjected to 5 min ischemia followed by 10 min reperfusion, where 1 mg/kg of SAR114646A applied as i.v. bolus 5 min prior to ischemia, decreased mortality to 0% compared to 80% under control conditions. In conclusion, SAR114646A is a multichannel blocker with improved water solubility, compared to amiodarone. In contrast to amiodarone, SAR114646A also blocks the K(+) channels I(to) and I(sus). A potent antiarrhythmic effect as observed in rats can also be expected in other animal models.

The effect of drugs with ion channel-blocking activity on the early embryonic rat heart

This study investigated the effects of a range of pharmaceutical drugs with ion channel-blocking activity on the heart of gestation day 13 rat embryos in vitro. The general hypothesis was that the blockade of the I(Kr)/hERG channel, that is highly important for the normal functioning of the embryonic rat heart, would cause bradycardia and arrhythmia. Concomitant blockade of other channels was expected to modify the effects of hERG blockade. Fourteen drugs with varying degrees of specificity and affinity toward potassium, sodium, and calcium channels were tested over a range of concentrations. The rat embryos were maintained for 2 hr in culture, 1 hr to acclimatize, and 1 hr to test the effect of the drug. All the drugs caused a concentration-dependent bradycardia except nifedipine, which primarily caused a negative inotropic effect eventually stopping the heart. A number of drugs induced arrhythmias and these appeared to be related to either sodium channel blockade, which resulted in a double atrial beat for each ventricular beat, or I(Kr)/hERG blockade, which caused irregular atrial and ventricular beats. However, it is difficult to make a precise prediction of the effect of a drug on the embryonic heart just by looking at the polypharmacological action on ion channels. The results indicate that the use of the tested drugs during pregnancy could potentially damage the embryo by causing periods of hypoxia. In general, the effects on the embryonic heart were only seen at concentrations greater than those likely to occur with normal therapeutic dosing.

The pharmaceutical pipeline for atrial fibrillation

Atrial fibrillation (AF) is associated with a significant burden of morbidity and increased risk of mortality. Beyond outstanding advances in catheter ablation procedures, antiarrhythmic drug therapy remains a corner-stone to restore and maintain sinus rhythm. However, potentially life-threatening hazards (proarrhythmia) and significant non-cardiac organ toxicity have made new drug development of prominent relevance. Multichannel blocking, atrial selectivity, and the reduction of the risk of adverse events have all constituted the main theme of modern antifibrillatory drug development. Dronedarone, an analog of amiodarone, has the unique characteristic of being the first antiarrhythmic drug demonstrated to reduce hospitalizations in AF. Dronedarone is associated with less systemic toxicity than amiodarone, although being less effective for sinus rhythm maintenance. Atrial selective agents have been developed to target ion channels expressed selectively in the atria. Among the most promising drugs of this class is vernakalant, which has been shown effective for the acute conversion of AF with small risk of proarrhythmia. Finally, increasing evidences support antiarrhythmic effectiveness of traditional non-antiarrhythmic drugs, such as renin-angiotensin system blockers, statins, and omega-3 fatty acids. In this article, we will focus on recent advances in antiarrhythmic therapy for AF, reviewing the possible clinical utility of novel antifibrillatory agents.

Benzofuran derivatives and the thyroid

Amiodarone and dronedarone are two clinically important benzofuran derivatives. Amiodarone has been used widely for treating resistant tachyarrhythmias in the past three decades. However amiodarone and its main metabolically active metabolite desethylamiodarone can adversely affect many organs, including the thyroid gland. Amiodarone-induced thyroid disorders are common and often present as a management challenge for endocrinologists. The pathogenesis of amiodarone-induced thyroid dysfunction is complex but the inherent effects of the drug itself as well as its high iodine content appear to play a central role. The non-iodinated dronedarone also exhibits anti-arrhythmic properties but appears to be less toxic to the thyroid. This review describes the biochemistry of benzofuran derivatives, including their pharmacology and the physiology necessary for understanding the cellular mechanisms involved in their actions. The known effects of these compounds on thyroid action are described. Recommendations for management of amiodarone-induced hypothyroidism and thyrotoxicosis are suggested. Dronedarone appears to be an alternative but less-effective anti-arrhythmic agent and it does not have adverse effects on thyroid function. It may have a future role as an alternative agent in patients being considered for amiodarone therapy especially those at high risk of developing thyroid dysfunction but not in severe heart failure.

Atrial-selective effects of chronic amiodarone in the management of atrial fibrillation

BACKGROUND

Although amiodarone is one of the most effective pharmacologic agents used in clinical management of atrial fibrillation (AF), little is known about its differential effects in atrial and ventricular myocardium.

OBJECTIVES

This study sought to compare the electrophysiological effects of chronic amiodarone in atria and ventricles.

METHODS

We compared the electrophysiological characteristics of coronary-perfused atrial and ventricular wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg/day for 6 weeks, n = 12).

RESULTS

Chronic amiodarone prolonged action potential duration (APD(90)) predominantly in atria compared to ventricles and prolonged the effective refractory period (ERP) more than APD(90) in both ventricular and atrial preparations (particularly in the latter) due to the development of postrepolarization refractoriness. Amiodarone reduced dispersion of APD(90) in both atria and ventricles. Although the maximum rate of increase of the action potential upstroke (V(max)) was significantly lower in both atria and ventricles of amiodarone-treated hearts versus untreated controls, the reduction of V(max) was much more pronounced in atria. Amiodarone prolonged P-wave duration more significantly than QRS duration, reflecting greater slowing of conduction in atria versus ventricles. These atrioventricular distinctions were significantly accentuated at faster activation rates. Persistent acetylcholine-mediated AF could be induced in only 1 of 6 atria from amiodarone-treated versus 10 of 10 untreated dogs.

CONCLUSION

Our results indicate that under the conditions studied, chronic amiodarone has potent atrial-predominant effects to depress sodium channel-mediated parameters and that this action of the drug is greatly potentiated by its ability to prolong APD predominantly in the atria, thus contributing to its effectiveness to suppress AF.

Drug therapy for atrial fibrillation

Atrial fibrillation (AF) is the most frequently diagnosed arrhythmia. Prevalence increases with age, and the overall incidence is expected to increase as the population continues to age. Choice of pharmacologic therapy for atrial fibrillation depends on whether or not the goal of treatment is maintaining sinus rhythm or tolerating atrial fibrillation with adequate control of ventricular rates. New antiarrhythmic drugs are being tested in clinical trials. Drugs that target remodeling and inflammation are being tested for their use as prevention of AF or as adjunctive therapy.

Pharmacologic targets for atrial fibrillation

Despite advances in treatment, atrial fibrillation (AF) remains the most common arrhythmia in humans. Antiarrhythmic drug therapy continues to be a cornerstone of AF treatment, even in light of emerging non-pharmacologic therapies. Conventional antiarrhythmic drugs target cardiac ion channels and are often associated with modest AF suppression and the risk of ventricular proarrhythmia. Ongoing drug development has focused on targeting atrial-specific ion channels as well as novel non-ionic targets. Targeting non-ionic mechanisms may also provide new drugs directed towards the underlying mechanisms responsible for AF and possibly greater antiarrhythmic potency. Agents that act against these new targets may offer improved safety and efficacy in AF treatment.

Novel pharmacological therapies for atrial fibrillation

PURPOSE OF REVIEW

Atrial fibrillation is a common yet difficult cardiac rhythm to treat. Limitations of the currently available medications, increasing complexity of atrial fibrillation patient populations and the prevalence of the condition have made new drug development crucial. Our understanding of the pathophysiology of atrial fibrillation has increased tremendously over the years. The importance of electrical remodeling and structural remodeling has been widely appreciated and has opened new avenues for pharmacological research.

RECENT FINDINGS

Novel ion channel blockers have targeted atrial-specific ion channels or a combination of ion channels in order to maximize efficacy and minimize proarrhythmic risk. Understanding of atrial fibrillation as a metabolically complex condition with activation of multiple signaling cascades has fuelled drug development in a new direction. Exciting new drugs inhibiting fibrosis, cellular hypertrophy and improving cell-cell communication may help treat chronic atrial fibrillation in the future.

SUMMARY

Continuing progress in our knowledge of the ionic and structural remodeling in atrial fibrillation will only accelerate the search for a safe antidote. In the future focal pharmacological modulation may help target specific targets in diverse populations. The potential of many of these pharmacotherapies, however, will need to be tested in large randomized trials before our faith in them is realized.

4-(3‘α15‘β-Dihydroxy-5‘β-estran-17‘β-yl)furan-2-methyl Alcohol: An Anti-Digoxin Agent with a Novel Mechanism of Action

The synthesis and some pharmacological properties of 4-(3'alpha-15'beta-dihydroxy-5beta-estran-17'beta-yl)furan-2-methyl alcohol (16) have been described. The compound was synthesized by reacting a synthetic 3alpha- benzyloxy-5beta-estr-15-en-17-one with the ethylene acetal of 4-bromo-2-furancarboxyaldehyde, followed by hydrolysis of the ethylene acetal and reduction of the aldehyde. Despite its resemblance to the structure of cardiac steroids (CS), 16 does not bind to the CS receptor on Na(+),K(+)-ATPase and does not increase the force of contraction of heart muscle. However, 16 inhibited the digoxin-induced increase in the force of contraction and arrhythmias in guinea pig papillary muscle and human atrial appendages. The steroid also inhibited digoxin-induced alteration in endocytosed membrane traffic, indicating a novel mechanism of action.

In Vivo and In Vitro Antiarrhythmic Effects of SSR149744C in Animal Models of Atrial Fibrillation and Ventricular Arrhythmias

SSR149744C (2-butyl-3-{4-[3-(dibutylamino)propyl]benzoyl}-1-benzofuran-5-carboxylate isopropyl fumarate) is a new noniodinated benzofuran derivative structurally related to amiodarone and dronedarone that is currently undergoing clinical trials as an antiarrhythmic agent. As SSR149744C exhibits electrophysiological and hemodynamic properties of class I, II, III, and IV antiarrhythmic agents, the aim of this study was to evaluate its acute intravenous (IV) or oral (PO) antiarrhythmic activities in in vitro and in vivo animal models of atrial and ventricular arrhythmias. In vagally induced atrial fibrillation (AF) in anesthetized dogs, SSR149744C (3 and 10 mg/kg IV) terminated AF in all 7 dogs and prevented reinduction in 4 out of 7 dogs; effective refractory periods of right atrium were dose-dependently and frequency-independently lengthened. In low-K+ medium-induced AF models, SSR149744C (0.1 to 1 microM) prevented AF in isolated guinea pig hearts in a concentration-dependent manner. At the ventricular level, SSR149744C (0.1 to 10 mg/kg IV and 3 to 90 mg/kg PO) prevented reperfusion-induced arrhythmias in anesthetized rats with a dose-effect relationship, and, at doses of 30 to 90 mg/kg PO, it reduced early (0-24 hours) mortality following permanent left coronary artery ligature in conscious rats. The present results show that SSR149744C is an effective antiarrhythmic agent in atrial fibrillation and in ventricular arrhythmias. Like amiodarone and dronedarone, its efficiency in these animal models of arrhythmias is likely be related to its multifactorial mechanism of action.

High affinity HERG K(+) channel blockade by the antiarrhythmic agent dronedarone: resistance to mutations of the S6 residues Y652 and F656

Pharmacological inhibition of human-ether-a-go-go-related gene (HERG) K(+) channels by structurally and therapeutically diverse drugs is associated with the 'acquired' form of long QT syndrome and with potentially lethal cardiac arrhythmias. Two aromatic amino-acid residues (Y652 and F656) on the inner (S6) helices are considered to be key constituents of a high affinity drug binding site within the HERG channel pore cavity. Using wild-type (WT) and mutant HERG channels expressed in mammalian cell lines, we have investigated HERG channel current (I(HERG)) blockade at 37+/-1 degrees C by dronedarone (DRONED), a non-iodinated analogue of the Class III antiarrhythmic agent amiodarone (AMIOD). Under our conditions WT I(HERG) tails, measured at -40 mV following activating pulses to +30 mV, were blocked with IC(50) values of approximately 59 and 70 nM for DRONED and AMIOD, respectively. I(HERG) inhibition by DRONED was contingent upon channel gating, with block developing rapidly on membrane depolarization, but with no preference for activated over inactivated channels. High external [K(+)] (94 mM) reduced the potency of I(HERG) inhibition by both DRONED and AMIOD. Strikingly, mutagenesis to alanine of the S6 residue F656 (F656A) failed to eliminate blockade by both DRONED and AMIOD, whilst Y652A had comparatively little effect on DRONED but some effect on AMIOD. These findings demonstrate that high affinity drug blockade of I(HERG) can occur without a strong dependence on the Y652 and F656 aromatic amino-acid residues.